This invention relates to a composite, liquid-cooled piston for internal combustion engines, particularly for medium-speed diesel engines, comprising a lower part which is connected by conventional means to an upper part which consists of ferrous material and a ring which is concentric with the upper part and is provided on the underside of the latter and bears on the opposite surface of the lower part and constitutes the radially inner boundary of an annular cooling passage which is disposed in the upper part behind the top land and at least part of the ring carrying portions of the upper part and open to the interfacial plane. The ring defines a central cooling chamber which is contained in the upper part and communicates with the cooling passage through radial coolant bores and is open to the interfacial plane, the cooling passage and the cooling chamber communicating with the coolant-circulating system by suitable coolant feed and discharge conduits which extend in the lower part substantially parallel to the longitudinal axis of the piston.
That composite piston is used in engines for very heavy duty and for an operation with heavy fuel oil. For this reason, cooling will always be required and will be effected as a forced-circulation cooling or as an injection cooling with shaker chambers as a standard design. The oil may flow through radially from the outside to the inside or in the opposite direction.
To minimize the dimension and weight of such composite pistons, the upper part of the piston consists of heat-resisting ferrous material, particularly forged steel, and the lower part consists of a eutectic aluminum-silicon piston alloy or of a ferrous alloy containing nodular graphite. The two parts are connected to one another by tie rods or screws or by soldered or welded joints, and cooling chambers are provided adjacent to the interfacial plane between the parts and serve to dissipate the heat which is generated at the top of the piston and cannot be dissipated otherwise
Such pistons have, as a rule, a relatively shallow combustion recess so that the highest head temperature, which is generally between 350.degree. and 400.degree. C. or even higher, will occur at the inclined outer rim of the recess owing to the shape of the jets of injected fuel. In that case, temperatures of about 240.degree. to 270.degree. C. may occur in the corresponding region of the inside surface of the cooling passage, which inside surface is wetted by cooling oil. These temperatures result in yellow to blue temper colors on the surface of the steel and are close to or above the flash point of commercial lubricating oils for diesel engines. The experience had with such pistons in use sometimes supports the belief that the cooling oil tends to coke very fast and forms an insulating layer of oil coke in the region of the cooling passage and that that layer reduces the cooling action so that the temperatures are much higher and the strength of the piston material and particularly its creep resistance will be reduced and the thermal deformation will be increased. It has been observed in several instances that this may result in permanent deformation. These recognitions have induced the design of oil-guiding rings for guiding the stream of oil to the hottest regions so that the velocity of flow is increased and the surface temperature is decreased in the regions in U.S. Pat. No. 4,175,502.
Basic model investigations of the cooling system of composite pistons have shown that a major portion of the heat is transferred to the upper portion of the cooling chamber and that appreciable quantities of heat are not transferred to the lower portion of the cooling chamber. As a result, the temperature drops to 110.degree. to 120.degree. C. adjacent to the first piston ring so that there is an undesired condensation of SO.sub.3, with all disadvantages involved therein, such as corrosion. Besides, the high concentration of heat on the outside surface of the head, resulting in temperatures of 300.degree. to 350.degree. C., will influence the stress concentration factor of the piston by thermal deformation.